Solving bugs in qubit_crosstalk routine

[lballerio]

this PR aims to solve two bugs in qubit_crosstalk routine:

	    flux_crosstalk = e.qubit_crosstalk(bias_point={"B1": 0.0},
	                                        targets=["B1", "B2"],
											flux_qubits=None 
											...)

1. if bias_point dictionary does not contain bias point for every target qubit _fit routine crashes since it is taking this dictionary as input for fitting., and a KeyError is given. I solved this bug by simply inserting default values for the missing qubits, i.e. the sweetspot in the calibration.json file of the platform. See fill_bias_points method.

2. if no list of qubits to flux (flux_qubits param) is given, the following error: TypeError: 'NoneType' object is not iterable arose because the params was not correctly configured. Hence we wrote drop_targets function for solving this issue: if no flux_qubit is given the function takes all qubits but the target ones.

Important

Bug n.2 needs also the Qibolab PR #1350 to be merged, otherwise the routine will still give error (not related to the ones reported here)

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.80%. Comparing base (155ad3b) to head (323bebe).

Files with missing lines	Patch %	Lines
...bocal/protocols/flux_dependence/qubit_crosstalk.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1327      +/-   ##
==========================================
- Coverage   97.83%   97.80%   -0.04%     
==========================================
  Files         132      132              
  Lines       10041    10049       +8     
==========================================
+ Hits         9824     9828       +4     
- Misses        217      221       +4     

Flag	Coverage Δ
unittests	97.80% <81.81%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...bocal/protocols/flux_dependence/qubit_crosstalk.py	97.05% <81.81%> (-1.38%)	⬇️

... and 1 file with indirect coverage changes

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[lballerio]

@alecandido should we put drop_targets as a method of QubitCrosstalkParameters class?

[alecandido]

It is an option, but not a mandatory one.

In particular, the call itself

    flux_qubits = (
        drop_targets(targets, platform)
        if params.flux_qubits is None
        else params.flux_qubits
    )

is quite related to the class.
But the content of drop_targets() is completely unrelated, since it is not making use of any information which is in the class.

Since, for the time being, it is meant to be used only internally (to this module), just prepend a _, to make it clear it is not intended for import. I.e. _drop_targets().

There are more or less three levels for top-level variables:

• internal (to the module), which are _-prefixed
• internal (to the qibocal package), just do nothing
• public (for qibocal users), which should be listed in __all__